1. Field of the Invention
This invention relates to electrical devices comprising conductive polymer compositions.
2. Introduction to the Invention
Conductive polymers are well known. They comprise a particulate conductive filler which is dispersed in, or otherwise held together by, an organic polymer. They can be used in circuits in which current passes through them, eg. in heaters and circuit protection devices, and in such use they may exhibit what is known as PTC (positive temperature coefficient) or ZTC (zero temperature coefficient) behavior. The term "PTC behavior" is usually used in the art, and is so used in this specification, to denote a composition which, in the operating temperature range, has an R.sub.14 value of at least 2.5 or an R.sub.100 value of at least 10, preferably both, and which preferably has an R.sub.30 value of at least 6, where R.sub.14 is the ratio of the resistivities at the end and the beginning of the 14.degree. C. temperature range showing the greatest increase in resistivity, R.sub.100 is the ratio of the resistivities at the end and the beginning of the 100.degree. C. temperature range showing the greatest increase in resistivity, and R.sub.30 is the ratio of the resistivities at the end and the beginning of the 30.degree. C. temperature range showing the greatest increase in resistivity. The term "ZTC behavior" is usually used in the art, and is so used in this specification, to denote a composition which does not show PTC behavior in the operating temperature range; thus the term is used to include (a) compositions which show no substantial change in resistivity over the operating temperature range (eg. from room temperature to 100.degree. C.), (b) compositions which show substantial increases in resistivity over the operating temperature range but still do not have R.sub.14, R.sub.30 or R.sub.100 values as specified above, (c) compositions which show substantial decreases in resistivity over the operating temperature range [often denoted NTC (negative temperature coefficient) compositions], and (d) compositions as defined in (a), (b) and (c) which exhibit PTC behavior at temperatures above the operating temperature range.
Documents describing conductive polymer compositions and devices comprising them include U.S. Pat. Nos. 3,861,029, 4,177,376, 4,237,441, 4,315,237, 4,352,083, 4,413,301, 4,426,339, and 4,459,473, the disclosures of which are incorporated herein by reference.